1. Field of the Invention
The present invention relates to the manufacturing method of semiconductor device, in particular, present invention relates to a light emitting device comprising a light emitting element formed on a plastic substrate. The invention also relates to an EL module in which ICs including a controller, or the like, is mounted with an EL panel. Note that, in this specification, the light emitting device includes the EL panel and for the EL module. Electronic equipment using the light emitting device is also included in the present invention.
Noted that in the present specification, the term “semiconductor device” generally indicates a device which is capable of functioning by utilizing semiconductor characteristics, and a light emitting device, an electro-optical device, a semiconductor circuit and an electronic device are all included in the semiconductor device.
2. Description of the Related Art
Recently, technology for forming TFTs (Thin Film Transistor) on a substrate has been greatly progressed, and its application to an active matrix display device is actively developed. In particular, a TFT using a polysilicon film have a higher field effect mobility (also referred to as mobility) than that of a conventional TFT using an amorphous silicon film, and thus, and is capable of high-speed operations. Therefore, a driver circuits that consist of TFTs using a polysilicon film is provided on the same substrate as pixels, and the development for controlling respective pixels is performed actively. Since driver circuits and pixels on one substrate are incorporated into an active matrix display device, there are various advantages such as reduction in the manufacturing cost, miniaturization of the display device, improvement in yield, and improvement in throughput.
In addition, an active matrix light emitting device (hereinafter, simply referred to as light emitting device), which has as a self-luminous element an EL element with a layer containing an organic compound as a light emitting layer, is actively researched. The light emitting device is also referred to as organic EL displays (OELDs) or organic light emitting diodes (OLEDs).
An EL element is self-luminous to have high visibility, and is optimal for making a display thin since a backlight like used for a liquid crystal display (LCD) is not required. Further, an angle of view has no limits. Therefore, a light emitting device using an EL element has thus come under the spotlight as a substitute display device for CRTs and LCDs.
An active matrix driving system for displaying an image by arranging a plurality of TFTs in each pixel and sequentially writing a video signal is known as one mode of a light emitting device using EL elements. The TFT is an indispensable element for realizing the active matrix driving system.
Conventional TFT was almost manufactured by using amorphous silicon. However, the TFT using amorphous silicon is low in electric field effect mobility, and cannot be operated at a frequency required to process the video signal. Accordingly, the TFT was used only as a switching element arranged in each pixel. A data line driving circuit for outputting the video signal to a data line, and a scanning line driving circuit for outputting a scanning signal to a scanning line were constructed by an external IC (driver IC) attached and mounted by TAB (Tape Automated Bonding) or COG (Chip on Glass).
However, when a pixel density is increased, a pixel pitch is narrowed. Accordingly, it is considered that there is a limit in a system of mounting the driver IC. For example, when UXGA (a pixel number of 1200×1600) is supposed, 6000 connecting terminals are required in an RGB color system even in simply estimating the number of connecting terminals. An increase in the number of connecting terminal causes an increase in generating probability of a contact defect. Further, the area (frame area) of a peripheral portion of a pixel section is increased and the compactness of a semiconductor device with and the design of an external appearance, as a display, are damaged. The necessity of the display device integrated with a driving circuit is clarified from such a background. The number of connecting terminals is greatly reduced and the frame area can be also reduced by integrally forming the pixel portion and the scanning line driving circuit and data line driving circuit on the same substrate.
As a means to realize the active matrix display device in which pixels and drive circuits are set on one substrate, a method for forming a TFT from a semiconductor film having crystalline structure, typically a polysilicon film, is proposed. However, even when the TFT is formed by using the polysilicon, its electrical characteristics are finally not equivalent to the characteristics of a MOS transistor formed in a monocrystal silicon substrate. For example, the electric field effect mobility of a conventional TFT is equal to or smaller than 1/10 in comparison with the monocrystal silicon. Further, the TFT using polysilicon has a problem that is dispersion is caused easily in its characteristics due to a defect formed in a boundary of a crystal grain.
In the light emitting device, at least a TFT functioning as a switching element and a TFT for supplying an electric current to an EL element are generally arranged in each pixel. A low off-electric current (Ioff) is required in the TFT functioning as the switching element while high driving ability (an on-electric current Ion), the prevention of deterioration due to a hot carrier effect and the improvement of reliability are required in the TFT for supplying the electric current to the EL element. Further, high driving ability (the on-electric current Ion), the prevention of deterioration due to the hot carrier effect and the improvement of reliability are also required in the TFT of the data line driving circuit.
Moreover, since the luminance of a pixel is determined by the ON current (Ion) of TFT which is electrically connected with an EL element and supplies current to the EL element without depending on the drive method, there is a problem dispersion is caused in luminance if ON current is not constant in case of displaying white on overall surface. For example, in case of adjusting luminance by light emitting time and performing 64 gray scales, the ON current of the TFT which is electrically connected with the EL element and supplies current to the EL element is dispersed 1.56% (= 1/64) from a fiducial point to shift one gray scale.